Field of the Invention
This invention relates generally to thermal emission sources and more particularly to electron emitters that exhibit high angular intensity and small virtual source size.
Description of the Prior Art
Free electrons that are generated in vacuum are used in many practical devices including Cathode Ray tubes, X-ray machines, electron microscopes, and e-beam lithography tools. Specialized electron emitters that can generate intense directed beams of electrons with specific properties have been developed for these applications. The properties that are important for these emitters are the angular intensity (i.e., the amount of electron current contained in a given emission angle), the emission cone angle size, the emitter virtual source size (i.e., the apparent area where the electron emission appears to be coming from), and the energy spread (i.e., the longitudinal variation in the electron energy).
Two common types of emitters are the thermal emitter and the field emitter, with each type of conventional emitter having their own advantages and disadvantages.
FIG. 1 shows one common method (the “thermal emitter” 10) used to create an electron emitter where a low work function material is heated to high temperature so that the thermal energy of electrons within the solid is high enough for some of the electrons to escape the material. A thermionic emitter is a common electron system implementation and has three main parts: (1) a hot material 20 acting as the emitter that is biased to a negative voltage, (2) an aperture 22 called a wehnelt that is placed in front of emitter and can be biased relative to the emitter, and (3) an anode electrode 24 with an aperture that has a positive voltage relative to the emitter and wehnelt. A beam 26 of electrons pass through the wehnelt and are accelerated through the anode to produce a beam of electrons with energy determined by the difference in voltage between the emitter and the anode. The negative bias on the wehnelt suppresses or turns back electron emission from areas other than within the wehnelt aperture.
As the beam is accelerated through the wehnelt and anode apertures, the beam forms a real cross-over 28. When used with an optical system the electrons appear to be coming from the cross-over location in front of the wehnelt and the apparent size of the emission area is determined by the size of the cross-over and is typically tens of micrometers in size. The amount of electron emission is dependent on the work function of the emissive material and the temperature the emitter is heated to. Lower work function materials produce more current while high temperature produce more current.
Thermal emitters have the advantage of emitting from a larger area, but cannot normally be placed past the wehnelt. When placed behind the wehnelt, much of the current is lost because only a small portion of the emitted particles makes it through the wehnelt aperture and is formed into the main electron beam.
To more tightly control the direction of electron emission at the source, a second common method (FIG. 2; the “field emitter” 30) is used to create an electron emitter where a low work function material is used in a high electric field so that the electrons within a solid can escape the material at a low thermal energy due to emission enhancement from the intense electric field at the surface of the emitter. An electron system implementation of this type of emitter commonly has three main parts: (1) an aperture 40 called a wehnelt or suppressor that can be biased relative to the emitter, (2) a material sharpened to a needle form 42 that protrudes through the wehnelt aperture and is biased to a negative voltage, and (3) an anode electrode 44 with an aperture 46 that has a positive voltage relative to the emitter and wehnelt. In this type of system, the emission is intense at the point of the needle emitter where the electric field is high enough to assist in the emission. This type of emitter is called a cold field emitter if operated near room temperature and a Schottkey emitter if operated at elevated temperatures. One main advantage of field emitters is that they typically have very small virtual source size due to the high field at the emission surface, often less than tens of nanometers in size. However, since the emission area is small these emitters have low current, low angular intensity, and a narrow emission angle
Conventional thermal emitters (FIG. 1) are not normally operated in a mode where the emitter extends in front of the wehnelt such as shown in FIG. 2 because all hot surfaces of the emitter that are exposed to the electric field emit electrons, thus resulting in electron emissions in a variety of directions rather than in a confined cone. The large amount of current produced outside the desired cone is problematic and reduces the performance of the emitter, thus having a negative impact on the key properties of the emitter. However, the field emitter of FIG. 2 also has a disadvantage because the emitting surface is only that small rounded tip of the sharpened emitter 42 where the electrons are emitted, thus resulting in a smaller emission. Accordingly, one makes a tradeoff between high emission but low directional control as with thermal emitters, or low emission and high directional control as with field emitters.
Accordingly, the need remains for an improved emitter that overcomes the problems inherent in these prior art designs.